This invention relates to an improved method for separating the solid particulate entrained in a gas stream and to an apparatus for effecting such separation. More particularly, the present invention relates to a method for separating solid particulate from a gas stream wherein a magnetic separator is employed and to the magnetic separator.
It is, of course, well known that the emission of solid particulate matter to the atmosphere is a hazard to both animal and plant life in the surrounding community. In this regard, it should be noted that the emission of solid particulate matter to the atmosphere commonly results from the combustion of carbonaceous fuels such as coal and oil during the production of electricity and in various chemical operations. Such emissions are also encountered during the crushing of stone and in various sand and gravel operations. Such emissions are also encountered in various agricultural operations such as grain elevators, feed mills, cotton gins and the like. The emission of the solid particulate to the atmosphere is also encountered during various mining and metal working processes such as in the mining of iron ore and the production of steel, the mining and production of copper and in the manufacture of aluminum. Such emissions are also encountered during the manufacture of various fertilizers, and the mining and processing of phosphate rock, during the manufacture and use of asphalt, in the cleaning of coal and in the production of carbon black. The actual effect on both the animal and plant life of a particular particulate will, of course, depend upon several factors such as the chemical and physical properties of the particulate and the particular animal or plant life effected thereby.
Heretofore, several methods and associated apparatus have been proposed for the purpose of separating particulate from various gas streams, thereby preventing their emission to the atmosphere. The more important methods are filtration, impingement, sedimentation, electrostatic precipitation, thermal precipitation and centrifugation. Magnetic separators have, however, been proposed and, indeed, may have found limited use for the purpose of separating solid particulate from a gas stream.
In general, each of these methods has been used with some degree of success. Each of the processes proposed heretofore, however, suffer from some disadvantage, and none of the processes proposed heretofore are particularly effective for separating submicron particulate matter, which particulate matter is now known to pose the greatest threat to the health and welfare of the surrounding community, especially with reasonable pressure drops or other operating conditions. For example, filtration can be used to effect the separation of relatively small particle size particulate. The pressure drop required to pass the media through the filter, however, increases rapidly as the pore size of the filter decreases. Moreover, even when reasonable pressure drops can be used at the beginning, the pressure drop required increases significantly as the amount of particulate separated increases. Impingement devices, on the other hand, are generally not effective for separating solid particulate having a particle size below about 2 microns unless the gas stream conveying the particles is travelling at a very high velocity. Similarly, sedimentation methods are not generally suitable for the separation of particles below about 5 microns in diameter. Electrostatic precipitation, on the other hand, is effective for the separation of particles as small as 0.01 microns but potential differences between about 12,000 and 30,000 volts are required to effect separation in this manner. Thermal precipitation will, of course, separate particles as small as 0.001 microns but here, temperature gradients as high as 3,000.degree. C./cm are often required. Cyclones, on the other hand, are not generally effective for the separation of particles smaller than about 5 microns.
As indicated previously, magnetic separators have been proposed heretofore. To date, however, these separators have not been widely used due partly to poor efficiencies in the submicron range and partly because other separating means must be used in combination therewith. In this regard, it should be noted that magnetic filters have been proposed heretofore but these devices suffer from substantially the same disadvantages as those indicated previously with respect to filtration generally. Also, the use of permanent magnets to attract or separate magnetic particulate has been proposed heretofore. Use in combination with other means to separate non-magnetic particulate, however, is generally required. Moreover, since permanent magnets have been proposed such devices are limited with respect to flexibility and significant equipment changes could be required if materials of significantly different magnetic moment were to be separated.
In light of the foregoing, then, it is believed that the need for an improved solid particulate separator which could be used to effect the separation of submicron particulate without requiring excessive pressure drops or extremely high gas velocities is readily apparent. Similarly, it is believed that the need for a magnetic separator capable of separating submicrons particulate and offering increased flexibility with respect to its ability to separate particulate having different magnetic moments is readily apparent.